Various modulation techniques are known to support radio communications. For example, constant envelope modulation techniques, such as frequency modulation (FM), are well known and understood. Non-constant envelope modulation techniques, such as .pi./4 differential QPSK, are also known.
Digital signalling techniques suitable for use with various modulation schemes are also known, such as .pi./4 differential QPSK (noted above) and 4 level FSK as used with FM. Although both techniques are well understood, present technology readily supports rapid introduction of 4 level FSK FM based radios, whereas .pi./4 differential QPSK based non-constant envelope radios pose a greater challenge. Although the various barriers to fielding a technologically and economically viable platform to support such signalling and modulation will no doubt exist in the near term future, users who require digital signalling will typically find 4 level FSK FM a more likely candidate for relatively immediate implementation.
Radio system users greatly desire immediate availability of digital signalling, in part for reasons of spectral efficiency, and in part to support various desired operating features. These same users, however, do not wish to invest in currently available technology at the expense of being either foreclosed from next generation advances, or at the expense of eliminating a currently acquired digital signalling system in favor of a next generation platform. In short, system users do not wish to acquire a 4 level FSK FM system to serve immediate needs, with the likely availability of .pi./4 differential QPSK radios in the future. At the same time, however, these same users want to realize the benefits of digital signalling now.
Accordingly, a need exists for some communications approach that will satisfy the current need for digital signalling, such as 4 level FSK FM, and yet viably accommodate likely future technologies, such as .pi./4 differential QPSK, in a cost effective manner.